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[Qemu-block] [PATCH 1/2] block: Open backing image in force share mode f

From: Fam Zheng
Subject: [Qemu-block] [PATCH 1/2] block: Open backing image in force share mode for size probe
Date: Fri, 15 Dec 2017 16:04:45 +0800

Management tools create overlays of running guests with qemu-img:

  $ qemu-img create -b /image/in/use.qcow2 -f qcow2 /overlay/image.qcow2

but this doesn't work anymore due to image locking:

    qemu-img: /overlay/image.qcow2: Failed to get shared "write" lock
    Is another process using the image?
    Could not open backing image to determine size.
Use the force share option to allow this use case again.

Cc: address@hidden
Signed-off-by: Fam Zheng <address@hidden>
 block.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 9a1a0d1e73..f236431da1 100644
--- a/block.c
+++ b/block.c
@@ -4593,10 +4593,11 @@ void bdrv_img_create(const char *filename, const char 
         back_flags = flags;
         back_flags &= ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
+        backing_options = qdict_new();
         if (backing_fmt) {
-            backing_options = qdict_new();
             qdict_put_str(backing_options, "driver", backing_fmt);
+        qdict_put_bool(backing_options, BDRV_OPT_FORCE_SHARE, true);
         bs = bdrv_open(full_backing, NULL, backing_options, back_flags,

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